An adequate supply of oxygen to tissues is essential in maintaining mammalian cell function and physiology. A deficiency in oxygen supply to tissues is a characteristic of a number of pathophysiologic conditions in which there is insufficient blood flow to provide adequate oxygenation, for example, ischemic disorders, cancer, and atherosclerosis. The hypoxic (low oxygen) environment of tissues activates a signaling cascade that drives the induction or repression of the transcription of a multitude of genes implicated in events such as angiogenesis (neo-vascularization), glucose metabolism, and cell survival/death. A key to this hypoxic transcriptional response lies in the transcription factors, the hypoxia-inducible factors (HIF). HIFs are disregulated in a vast array of cancers through hypoxia-dependent and independent mechanisms and expression is associated with poor patient prognosis.
HIFs consist of an oxygen-sensitive HIFα subunit and a constitutively expressed HIFβ subunit. When HIFs are activated, the HIFα and HIFβ subunits assemble a functional heterodimer (the a subunit heterodimerizes with the 3 subunit). Both HIFα and HIFβ have two identical structural characteristics, a basic helix-loop-helix (bHLH) and PAS domains (PAS is an acronym referring to the first proteins, PER, ARNT, SIM, in which this motif was identified). There are three human HIFα subunits (HIF-1α, HIF-2α, and HIF-3α) that are oxygen sensitive. Among the three subunits, HIF-1α is the most ubiquitously expressed and induced by low oxygen concentrations in many cell and tissue types. HIF-2α is highly similar to HIF-1α in both structure and function, but exhibits more restricted cell and tissue-specific expression, and might also be differentially regulated by nuclear translocation. HIF-3α also exhibits conservation with HIF-1α and HIF-2α in the HLH and PAS domains. HIF-1β (also referred to as ARNT-Aryl Hydrocarbon Receptor Nuclear Translocator), the dimerization partner of the HIFα subunits, is constitutively expressed in all cell types and is not regulated by oxygen concentration.
PD-1 (Programmed cell death protein-1) is a cell surface co-inhibitory receptor expressed mainly on T cells and B cells. PD-1 has two known ligands: PD-L1 and PD-L2. The PD-1/PD-L1 signaling axis plays an important role in the negative regulation of the immune system to prevent autoimmunity and promote self-tolerance. PD-L1 has been thought to be the principle mediator of PD-1 dependent immunosuppression.
PD-L1 is expressed in many human cancers and is associated with a poor prognosis for patients. Tumor-infiltrating lymphocytes from patients with cancer typically express PD-1 and display impaired antitumor functionality. Preclinical studies demonstrate that blockage of the interaction between PD-1 and PD-L1 can enhance T-cell function and mediate anti-tumor activity. Monoclonal antibodies targeting PD-1 or PD-L1 are under development for the treatment of cancer. One such PD-1 targeting agent, nivolumab (Opdivo, Bristol-Myers Squibb), produced complete or partial antitumor responses in multiple diseases, including non-small-cell lung cancer, melanoma and renal-cell cancer in a clinical trial, and was approved by the FDA to treat metastatic melanoma in 2014. There are at least 7 mAbs that target the PD-1/PD-L1 interaction being evaluated in clinical studies that hold promise for the immunotherapeutic treatment of various malignancies.
Preclinical and early clinical studies demonstrated that the combination of anti-PD-1/PD-L1 agents with other targeted agents could increase anti-tumor efficacy. Simultaneous treatment with anti-PD-1 and anti-VEGFR2 mAbs inhibited tumor growth synergistically in a preclinical murine colon cancer model. The most recent phase I clinical trial data showed that combining nivolumab with one of the current standard of care VEGFR tyrosine kinase inhibitors (TKIs), either sunitinib or pazopanib, significantly increased the overall response rate compared with the single agent alone in patients with renal cancer.
CTLA-4 (Cytotoxic T-lymphocyte-associated antigen 4) is a member of the immunoglobulin superfamily, which is expressed on the surface of T cells. CTLA-4 transmits an inhibitory signal to T cells by outcompeting the T-cell co-stimulatory molecule CD28 for B7 ligands (CD80 and CD86) on the surface of antigen-presenting cells with higher affinity and avidity. In preclinical studies, blockade of CTLA-4 led to a significant in T-cell proliferation and interleukin-2 production.
In addition to the suppression of the effector T cells, CTLA-4 also increases the function of immunosuppressive T regulatory T cells (Tregs). CTLA-4 deficiency in Tregs was shown to diminish the suppressive capacity of Tregs, and CTLA-4 blockade has been shown to deplete intratumoral Tregs in preclinical models.
Based on these preclinical findings, two antibodies that block CTLA-4 in humans, ipilimumab and tremelimumab, have been tested in the clinic and have demonstrated significant durable responses in a broad of spectrum of malignancies. The FDA has approved both ipilimumab and tremelimumab for the treatment of melanoma.